Many catalytic processes exist for the polymerization or copolymerization of olefins such as ethylene and propylene. These processes have traditionally utilized a Ziegler-Natta catalyst system. These catalyst systems contain a transition metal compound (typically a titanium, zirconium, or vanadium halide or alkoxide) and a main group metal alkyl (usually an aluminum alkyl). The Ziegler-Natta catalyst systems are heterogeneous and possess a number of different active catalyst sites. Each different active site has different characteristics and produces a different polymer, and as a result, Ziegler-Natta catalyst systems produce polyolefins with broad molecular weight distributions and copolymers with broad compositional distributions.
Recent developments in the field of olefin polymerization have focused on the use of transition metal compounds having at least one n-bound cyclopentadienyl ligand. The cyclopentadienyl ligand can be substituted or unsubstituted, and generally includes fused ring derivatives such as indenyl and fluorenyl. These cyclopentadienyl transition metal compounds are often referred to as metallocenes, though the term was initially used to describe biscyclopentadienyl compounds such as dicyclopentadienyliron (ferrocene).
Olefin polymerization systems using metallocenes differ from Ziegler-Natta catalyst systems in important ways. With metallocene catalysts there is generally only one catalytically active species responsible for the polymerization of the monomers. The metallocenes, therefore, produce uniforms chains of polymer having narrower molecular weight distributions and narrower compositional distribution. Metallocene catalysts are also typically much more active on a weight basis than Ziegler-Natta catalysts. Metallocene catalysts can be 10 to 1,000 times more active than the best Ziegler-Natta catalysts.
Metallocene catalysts are often classified into two separate groups, those possessing one cyclopentadienyl ligand, and those possessing two cyclopentadienyl ligands. The monocyclopentadienyl metallocenes are generally known in the art as good styrene polymerization catalysts and poor olefin polymerization catalysts, whereas biscyclopentadienyl metallocenes are generally known in the art as good olefin polymerization catalysts and poor styrene polymerization catalysts. Representative examples of these various catalysts are disclosed in U.S. Pat. Nos. 4,978,730; 5,023,222; 5,045,517; 5,066,741; 5,196,490; and 5,340,892 disclosing monocyclopentadienyl metallocenes. Examples of biscyclopentadienyl metallocenes are disclosed in U.S. Pat. Nos. 4,404,344; 4,542,199; 4,752,597; 5,198,401; 5,278,119; and 5,453,475.
Despite the utility of the catalysts disclosed above, a need still exists to discover more useful and efficient catalysts to polymerize olefins.